1. Field of the Invention
The present invention relates to control of an image density in an image forming apparatus which forms an electrostatic latent image by exposing a surface of a photosensitive member with a light beam and thereafter visualizes the electrostatic latent image with toner to thereby create a toner image.
2. Description of the Related Art
In this type of image forming apparatus, the image densities of toner images become sometimes different because of differences among the individual apparatuses, a change with time or fatigue of a photosensitive member and toner, a change in temperature and/or humidity around the apparatus, etc. Noting this, techniques for stabilizing an image density have been proposed which require to calculate optimal values of a charging bias and a developing bias, namely, an optimal charging bias and an optimal developing bias and set the charging bias and the developing bias respectively to the optimal charging bias and the optimal developing bias. For example, according to the invention described in Japanese Patent Application Unexamined Gazette No. H10-239924, predetermined toner images are formed on a photosensitive member as patch images while changing a charging bias and a developing bias and the image density of each reference patch is measured. Based on thus measured values, an optimal charging bias and an optimal developing bias for achieving target densities in two gradation levels, i.e., a lower-density side target density and a higher-density side target density, are determined.
However, with an approach requiring to form a plurality of patch images while changing a charging bias, since a photo-induced discharge characteristic of a photosensitive member changes with a change in charging bias and an electric potential in an exposed area largely changes, although an optimal developing bias for attaining a higher-density side target density for instance can be calculated, it is sometimes impossible to calculate an optimal charging bias which is for attaining a lower-density side target density. The opposite situation as well can occur. Thus, with a conventional apparatus noting a charging bias and a developing bias as density adjustment factors for adjusting the image densities of toner images, it is difficult to stabilize the image densities of toner images in a wide density range.
Further, in this type of image forming apparatus, since an abutting member, such as a cleaning part for clearing residual toner remaining on a surface of a photosensitive member, abuts on the photosensitive member, the film thickness of a carrier transport layer decreases in accordance with an increase in cumulative number of revolutions or the like of the photosensitive member. This changes an electric field upon the carrier transport layer, and hence, changes a photo-induced discharge characteristic of the photosensitive member because of changed mobility of carrier. The change in photo-induced discharge characteristic of the photosensitive member also changes a development xcex3 characteristic. Such changes in characteristic due to the change in film thickness of the photosensitive member may further invite the following problems.
The first problem is that even though the density adjustment factors are set to optimal values in advance and the image densities of toner images are accordingly controlled to target densities, as the photo-induced discharge characteristic of the photosensitive member changes due to the change in film thickness of the photosensitive member, the optimal values of the density adjustment factors as well change. A result of this is a deviation of the image densities of the toner images from the target densities.
Meanwhile, so-called patch sensing method has been used frequently for optimization of the density adjustment factors. During patch sensing, patch images are created while modifying the density adjustment factors within a programmable range determined in advance, and optimal values of the density adjustment factors are determined based on the image densities of the respective patch images. At this stage, the photo-induced discharge characteristic of the photosensitive member may sometimes change due to a change in film thickness of the photosensitive member, whereby the optimal values of the density adjustment factors exceed the programmable range. When this occurs, it becomes impossible to calculate the optimal values of the density adjustment factors, and hence, control the image densities of the toner images to the target densities.
In addition, since the density adjustment factors are controlled so that the image densities of the toner images are always the target densities in the conventional image forming apparatus as described above, the apparatus can not flexibly respond to a demand from a user, and therefore, has a problem in terms of versatility. This is because a user in some cases wishes to create an image at a density which is different from an image density, i.e. reference target density set as a default value by a manufacturer. For instance, it is sometimes desirable to develop a thin line darker than the reference target density in a CAD view or an image with a great number of characters. Further, when one wishes to confirm the overall layout, the hue or the like of a print image before actually obtaining a final print output, it may be desirable to lower the density of the entire image to the extent that confirmation will not be hampered and accordingly suppress the consumption of toner. Despite this, since an image density is controlled by means of the reference target density set in advance in the conventional image forming apparatus, it is difficult to appropriately respond to such a demand from a user.
A principal object of the present invention is to provide an image forming apparatus and an image forming method with which it is possible to stabilize the image density of a toner image in a wide density range in an easy and reliable manner.
Another object of the present invention is to provide an image forming apparatus and an image forming method with which it is possible to reliably stabilize the image density of a toner image regardless of a change in film thickness of a photosensitive member.
Still another object of the present invention is to provide an image forming apparatus and an image forming method with which it is possible to properly control the image density of a toner image while flexibly responding to a demand from a user.
In fulfillment of the foregoing object, an image forming apparatus and method are provided and are particularly well suited to density adjustment of a toner image.
According to a first aspect of the present invention, one piece of correlation data, which belongs to a product set of lower-density side correlation data and higher-density side correlation data, is selected as optimal values of a plurality of density adjustment factors. The lower-density side correlation data are between said plurality of density adjustment factors and for forming a toner image at a lower-density side target density, and the higher-density side correlation data are between said plurality of density adjustment factors and for forming a toner image at a higher-density side target density which is higher than said lower-density side target density.
According to a second aspect of the present invention, a density adjustment factor influencing an image density of a toner image is set to a value which corresponds to a film thickness of a photosensitive member, to thereby control the image density of a toner image formed on a photosensitive member.
According to a third aspect of the present invention, a density adjustment factor is modified in accordance with a necessity so as to change an image density of a toner image to an image density which is deviated from a reference target density.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawing. It is to be expressly understood, however, that the drawing is for purpose of illustration only and is not intended as a definition of the limits of the invention.